The economic and technical challenges are generally known, which exist in collecting the freely accessible solar energy. Technically such a solution has to manage big and swift variations of temperature, varying air humidity and mechanical stress. The device, i.e. the solar energy collector, should also in an economically justifiable way move the solar energy from a relatively large area to a point, where it can be used.
Solar cooling has increasingly begun to be used as an efficient way of eliminating CO2-emissions caused by conventional cooling plants. However, a disadvantage associated with solar cooling systems is that they often are substantially more expensive than common cooling plants. In a solar cooling system, for example chemical heat pumps can be used.
The principle of the function of the chemical heat pump is well known, see for example U.S. Pat. Nos. 5,440,889, 5,056,591, 4,993,239, 4,754,805 and the published International patent applications WO 94/21973, WO 00/31206, WO 00/37864 and WO 2005/054757. In a chemical heat pump an active substance is provided that performs the very process of the heat pump and that works together with a volatile medium, the absorbent, which usually is a dipolar liquid, in most cases water. As the working active substance can, according to the prior art, either a solid substance, a liquid substance or a “hybrid substance” be used. By “solid” active substance is meant that the substance all the time, during the whole process and all cycles remains in a solid state, i.e. both with and without a volatile medium absorbed therein. By a “liquid” active substance is meant that the substance all the time, during the whole process and all cycles, remains in a liquid state, i.e. both with and without a volatile medium absorbed therein. By a “hybrid” substance is meant that the active substance during the process in the heat pump is alternating between a solid state and a liquid state.
For a solid active substance, advantages are obtained that include that the cooling temperature in the system in which the heat pump is incorporated remains constant during the whole discharging process and that a relatively large storage capacity can be obtained. A typical value of the storing capacity for a solid substance using water as the absorbent, taken as cooling energy, is about 0.3 kWh/l substance. Another advantage associated with the use of a solid substance is that no moving components are required in the system. Heat is supplied to or drawn from the substance through a lamellar heat exchanger or a plate heat exchanger that is in a homogeneous contact with the substance. Hence, in the chemical heat pump described in the cited patent application WO 00/31206 no moving components are provided on the process side. The disadvantage associated with a solid substance is the limited power that can be obtained due to the generally low heat conductivity of solid substances. In the same patent application, among other things, a method is described for solving the problem associated with the bad heat conductivity of solid substances and the low power/efficiency resulting therefrom. The method includes that the solid substance is silted up in the sorbate to form a slurry having such a consistency that it can be easily filled around or into a heat exchanger. The amount of sorbate in the slurry should exceed the concentration of sorbate that will later exist in the discharged state of the heat pump. Thereafter, when the substance is charged it obtains a final sintered shape, a so called matrix, which is not dissolved in the normal absorption of sorbate in the operation of the heat pump.
For the use of a liquid substance the advantage of a high power is obtained since the substance can be sprayed over the heat exchanger in both the charging and the discharging processes and hence be efficiently cooled and heated, respectively. The disadvantage associated with a solid substance is that the cooling capacity decreases as a function of the dilution of the absorbent. Actually, it limits strongly the operating interval within which the substance can be used, this in turn reducing the storage capacity, taken as above as cooling energy per litre substance. Most of the liquid substances for use in chemical heat pumps are solutions of strongly hygroscopic inorganic salts in preferably water and similarly water is used as the absorbent. This gives another limitation due the fact that the dissolved substance cannot be allowed to crystallize. Crystallization creates problems in spray nozzles and pumps.
By using a so called hybrid substance several of the advantages associated with solid and liquid systems can be combined, see the International patent application WO 00/37864 cited above. The chemical heat pump disclosed in this patent application operates according to a special procedure that can be called the hybrid principle, the hybrid method or the hybrid process. In that process, the substance exists both in a solid and a liquid state during the process, the solid phase being used for storing energy, with as large an energy density as in solid systems whereas the heat exchange to and from the substance is only made in the liquid phase of the substance with as large an efficiency as in common liquid systems. Only the liquid phase is used for heat exchange to the surroundings. A condition thereof is that the solid and liquid phases can be kept separated during the process. A separation can be obtained by filtering using a separating means of a suitable kind, such as a net or a filter or in some other way. The liquid phase, often called the “solution”, is pumped and sprayed over a heat exchanger. As in the case of systems using only a solution, i.e. with a substance that all time is liquid, it is important that the pumps, valves and spray nozzles of hybrid systems are not blocked by crystals in the circulation path.
Thus generally, the solid system has in this regard an apparent advantage since it does not require any pumps, valves and spray nozzles.
In FIG. 1a a chemical heat pump is generally shown in a schematic way, the heat pump designed for producing cooling or heat and working according to the hybrid process described in the cited International patent application WO 00/37864. The heat pump includes a first container 1 or accumulator including a more or less dissolved substance 2 that can exothermically absorb or endothermically desorb a sorbate. The first container 1 is connected to a second container 3, also called condenser/evaporator, through a pipe 4. The second container 3 works as a condenser for condensing gaseous sorbate 6 to form liquid sorbate 5 during endothermic desorption of the substance 2 in the first container 1 and as an evaporator of liquid sorbate 5 to form gaseous sorbate 6 during exothermal absorption of the sorbate in the substance 2 in the first container 1. The substance 2 in the accumulator 1 is in heat conducting contact with a first heat exchanger 7 located therein which can in turn through a liquid flow 8 be supplied with heat from or deliver heat to the surroundings. The liquid 5 in the evaporator/condenser part 3 is similarly in a heat conducting contact with a second heat exchanger 9 located therein to or from which heat can be supplied or delivered from or to the surroundings, respectively, through a heat flow 10. In order that the heat pump will work according to the hybrid principle the first heat exchanger 7 together with the substance 2 in the solid state thereof is enclosed in a fine-meshed net or filter 11. Solution that is the liquid state of the substance exists in the lower portion of the accumulator 1 and is there collected in a free space 12 located beneath the first heat exchanger 7. From this space solution can through a conduit 13 and a pump 14 be sprayed over the first heat exchanger 7.
To sum up, the following is true:                In a system working with a solid substance a constant cooling temperature is obtained since the reaction occurs between two phase states of the substance. Both of these two phase states are solid and maintain, in a transformation from one of the states to the other state, a constant reaction pressure of the absorbent. The reaction pressure remains constant until all of the substance has been transformed from the first state to the second state. The disadvantage of the system is the very low heat conductivity and the low power resulting therefrom. Its advantages include that it works without any moving parts, has a high storage capacity and a constant reaction pressure.        In a system working with a hybrid substance the first phase is, when the absorbent is absorbed by the substance, i.e. in the discharge process, solid whereas the second phase is liquid and then in the same way as above, a constant reaction pressure of the absorbent is maintained. The substance will then successively continuously change from a solid to a liquid state at the same time as a constant cooling temperature is obtained. The process continues with a constant reaction pressure until all of the substance has changed from its solid to its liquid state. In the same way the reaction pressure is constant in the charging process when the substance changes from a liquid to a solid state. The storage capacity and the reaction pressure are equivalent to those for a solid substance. The method used in systems working with a hybrid substance in order to obtain a high power is to work with solutions in the same way as in a system working with a liquid substance. Liquid is pumped from the substance container through a system for separating crystals to a spraying system by which the solution is sprinkled over the heat exchanger that forms a separate unit in the reactor.        
A heat pump driven by solar energy is disclosed in U.S. Pat. No. 4,231,772, in which a chamber working as an accumulator is connected to or itself works as a solar energy collector.